Rubber polymers and acrylic polymers as viscoelastic materials are widely adopted typically to sheets and films, or to pressure-sensitive adhesives and cushioning materials.
When adopted to pressure-sensitive adhesives, rubber pressure-sensitive adhesives are prepared by incorporating various additives such as tackifiers to elastomer materials such as natural rubbers or synthetic rubbers. Such rubber pressure-sensitive adhesives are relatively not so selective with respect to adherends and thereby exert satisfactory adhesiveness even when applied to nonpolar plastics (e.g., polyolefinic resins such as polypropylenes and polyethylenes) to which acrylic pressure-sensitive adhesives may not exhibit adhesiveness. In contrast, the acrylic pressure-sensitive adhesives are superior in adhesiveness to adherends having relatively high polarity, such as metals, as compared to rubber polymers and, in addition, are more advantageous typically in transparency, durability, and thermal stability than the rubber pressure-sensitive adhesives.
As is described above, either of the rubber pressure-sensitive adhesives and acrylic pressure-sensitive adhesives have both advantages and disadvantages, and these pressure-sensitive adhesives are used depending on the purpose or intended use. Exemplary uses of pressure-sensitive adhesives include a pressure-sensitive adhesive tape or sheet (hereinafter such a “tape or sheet” is also simply referred to as a “tape” or “sheet”); and a label. When a double-sided pressure-sensitive adhesive tape, for example, is adopted to joining of two or more articles with each other, demands are often made to join articles (adherends) made from different materials with each other through the pressure-sensitive adhesive. In these cases, it is not rare that one of the adherends is made from a nonpolar material such as a polyolefin, and the other adherend is made from a polar material such as a metal. In this case, neither rubber pressure-sensitive adhesives nor acrylic pressure-sensitive adhesives can provide satisfactory joining between the two adherends of different types through the pressure-sensitive adhesive.
To solve these problems, demands have been made to provide pressure-sensitive adhesives having both advantages of rubber pressure-sensitive adhesives and advantages of acrylic pressure-sensitive adhesives. Typically, there are known pressure-sensitive adhesives as blends of a rubber viscoelastic material and an acrylic polymer (see Patent Document 1 and Patent Document 2). The use of such blended pressure-sensitive adhesives improves the adhesiveness to adherends which are hard to adhere through acrylic pressure-sensitive adhesives. These blended pressure-sensitive adhesives, however, have a microphase separation structure because of poor miscibility between the rubber viscoelastic material and the acrylic polymer, and the microphase separation structure may cause insufficient transparency and staining of the adherend and may vary the performance of the adhesives when the adhesives are stored for a long duration from the production.
Another possible solution is a technique of preparing a film of a rubber pressure-sensitive adhesive and a film of an acrylic pressure-sensitive adhesive; and affixing the two films with each other. This technique, however, requires a complicated process to produce the pressure-sensitive adhesive, because the technique needs the steps of forming a film of rubber pressure-sensitive adhesive according to a procedure, forming a film of acrylic pressure-sensitive adhesive according to another procedure, and affixing the two films. Independently, there are demands based on recent environmental issues to reduce organic solvents and to prevent global warming due to carbon dioxide (CO2). However, filming of an acrylic pressure-sensitive adhesive and a rubber pressure-sensitive adhesive, if to be performed without using solvents so as to meet the above demands, needs two filming processes of different types. Specifically, an acrylic pressure-sensitive adhesive can be filmed substantially without using any solvent and without the need of a drying step when a polymerization process by the action of ultraviolet rays (UV polymerization process) as disclosed typically in Patent Document 3 is adopted; but a rubber pressure-sensitive adhesive is difficult to be filmed by the UV polymerization process and should be, for example, converted into a hot melt pressure-sensitive adhesive or aqueous dispersion in order to form a film without using solvents.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. H02 (1990)-47182    Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. H02 (1990)-45580    Patent Document 3: U.S. Pat. No. 4,181,752